The increased interest in wireless power transfer technology is evident both from the technical and consumer perspectives. Improved system efficiencies due to the emergence of resonant transfer of power have engendered the rapid increase of applications that are emerging for this technology.
These known power receivers have a power transfer efficiency that is unsatisfactorily for modern appliances, in which power consumption issues are very strict.
Wireless Power Systems (WPS), such as but not limited to that which is described by the Wireless Power Consortium (WPC) in the Qi standard (System description wireless power transfer Volume I: Low power, Part 1: Interface definition, Wireless Power Consortium v.1.0.1 Oct. 2010, incorporated by reference), use coupled electromagnetic (EM) fields from a primary subsystem (power transmitter) to transfer power through a nonconductive medium. The field is captured in a secondary subsystem and converted to useable energy.
A high-level block diagram of a Wireless Power System for powering a mobile device is depicted in FIG. 1. The shown power transmitter comprises two main functional units, namely a power conversion unit and a communications and control unit. The diagram explicitly shows the primary coil as the magnetic field generating element of the power conversion unit. The control and communications unit regulates the transferred power to the level that the magnetically coupled power receiver requests. A base station may contain multiple power transmitters in order to serve multiple mobile devices simultaneously. The wireless power system shown in the diagram typically comprises all other functionality of the base station, such as input power provisioning, control of multiple power transmitters, and user interfacing.
A power receiver comprises a power pick-up unit and a communications and control unit. Similar to the power conversion unit of the transmitter, the secondary coil is the magnetic field capturing element of the power pick-up unit. The communications and control unit regulates the transferred power to the level that is appropriate for the load connected to the output of the power receiver.
An important exemplary load to be powered is a battery pack that requires charging. Low power devices that use batteries are of real concern when using a WPS due to the adverse relation between increased temperatures with battery safety and performance. For these reasons, power receivers are equipped with a communications and control unit in order to cooperate to regulate the transferred power to the desired level. For this purpose, the power receiver communicates its power needs on a regular basis and continuously monitors the power transfer to ensure that limits imposed by the standard are not violated. If a violation occurs anyway, the power transmitter may abort the power transfer. This means that the power receiver communicates the difference between a desired set point and the actual set point to the power transmitter, which adjusts the primary coil current so as to reduce the error towards zero.
Exemplary electric diagrams of power transmitters are shown in FIGS. 2 and 3. They typically comprise a switching stage (half-bridge or full-bridge) that drives a L-C circuit that includes the primary coil LP and a corresponding primary capacitor CP fixing the resonance capacitance of the L-C circuit. The primary coil LP is used for supplying power to the power receiver and for receiving data signals (typically at higher frequency than the frequency of the power supply) generated by the communications and control unit of the power receiver.
An exemplary functional block diagram of a power receiver is shown in FIG. 4. The power pick-up unit on the left-hand side of FIG. 4 comprises the analog components of the power receiver, namely:                a dual resonant circuit, comprising a secondary coil plus series and parallel capacitances to enhance the power transfer efficiency and enable a resonant detection;        a rectification circuit that provides full-wave rectification of the AC waveform, using e.g. four diodes in a full-bridge configuration, or a suitable configuration of active components. The rectification circuit may perform output smoothing as well. In this example, the rectification circuit provides power to both the communications and control unit of the power receiver and the output of the power receiver;        a communications modulator: on the DC side of the power receiver, the communications modulator typically comprising a resistor in series with a switch. On the AC side of the power receiver, the communications modulator typically comprises a capacitor in series with a switch (not shown in FIG. 4);        an output disconnect switch, which prevents current from flowing to the output when the power receiver does not provide power at its output. In addition, the output disconnect switch prevents current back flow into the power receiver when the power receiver does not provide power at its output. Moreover, the output disconnect switch minimizes the power that the power receiver draws from the power transmitter when a power signal is first applied to the secondary coil;        a rectified voltage sense.        
The communications and control unit on the right-hand side of FIG. 4 comprises the digital logic part of the power receiver. This unit executes the relevant power control algorithms and protocols, drives the communications modulator, controls the output disconnect switch, and monitors several sensing circuits, in both the power pick-up unit and the load. An example of a sensing circuit in the load is a circuit that measures the temperature of, e.g., a rechargeable battery.
Power receiver designs that differ from the example functional block diagram shown in FIG. 4 are possible. For example, an alternative design includes post-regulation of the output of the rectification circuit (e.g., using a buck converter, battery charging circuit, power management unit, etc.).
The dual resonant circuit of the power receiver comprises a secondary coil LS and two resonant capacitances CS and CD, as shown by way of example in FIG. 5. The secondary resonant capacitance CS is used to enhance power transfer efficiency. The purpose of the auxiliary resonant capacitance CD is to enable a resonant detection method. FIG. 5 illustrates a dual resonant circuit. The switch in the dual resonant circuit is optional. If the switch is not present, the capacitance CD has a fixed connection to the secondary coil.
An exemplary electric diagram of a power receiver that includes a communications modulator connected to a secondary coil LS, a rectification circuit composed of a diode bridge, suitable for charging a Lithium ions battery is shown in FIG. 6. Data are transmitted from the power receiver back to the power transmitter with a “back-scattering” technique by switching a R-C load (composed of the resistor R and of the two communication modulating capacitances CCM) referred to a common ground node.
An alternative electric diagram of a power receiver is shown in FIG. 7. The “back-scattering” technique is implemented by switching a resistive load RCM referred to a common ground node. The rectified voltage VR is provided in input to a voltage regulator, for example a buck converter, that generates a regulated voltage that may be used to charge a battery. Information about the charging state of the battery is transmitted back to the power transmitter with a back scattering technique while the battery is being charge. Typically, exemplary time graphs of the charge voltage of the battery and of the charge current absorbed therefrom are as shown in FIG. 8.